The present invention is related to digital logic circuits and, in particular, to MOS input transition detector circuits.
Input transition, sometimes called address transition, detectors have been widely used in integrated circuits. These detectors generate a pulse upon the transition of a signal at an input terminal of the circuit. There are various versions of such circuits but functionally the circuit is simply a circuit, as shown in FIG. 1, which has an EXCLUSIVE OR logic gate and a delay block. An EXCLUSIVE OR gate by definition generates a logic "1" signal when there is one and only one logic "1" signal at one of its input terminals. Stated differently, a logic 1 occurs when the two input signals are different. Thus, the circuit in FIG. 1 generates a logic 1 at its output when there is a transition at the input terminal from either logic 1 to logic 0, or logic 0 to logic 1. At this point the two input terminals to the EXCLUSIVE OR gate are different. The delay block puts in a time delay .tau. after which input terminals of the EXCLUSIVE OR gate have the same input states and causes the output terminal to return to a 0 state. Hence a pulse is generated.
The problem with this type of circuit is that there is difficulty in controlling the delay pulse width. Due to variations in the manufacturing process and operating conditions, the delay pulse width may vary widely. In many applications, such variations in a delay pulse width are tolerable. But in other cases the pulse width must be controlled.
The present invention provides for a circuit which detects input transitions at a plurality of input terminals and generates a single pulse which has a time delay which may be controlled with precision. Only a small number of transistors and other integrated circuit device elements are used to implement the circuit to achieve the benefits of a savings in valuable integrated circuit space. Furthermore, with the circuit according to the present invention, the generated pulse may be distributed in a network throughout the integrated circuit.